This invention relates generally to lifts for watercraft and more particularly concerns buoyancy tanks for boat lifts and the like.
Corrosion is a major concern in the manufacture and maintenance of boat lifts. Typically, boat lift buoyancy tanks are encompassed by steel which secures the tanks and permits connection of other components to the lift. In use, at least parts of the steel are always submerged and constantly exposed to a corrosive environment which leads to shortened lift life. One method used by the industry to deal with this problem is the use of steel inserts, such as specially threaded nuts, which are molded into plastic parts. Since the steel must be fixed to the lift in the specific locations and orientations dictated by the inserts, design flexibility is limited. Furthermore, these inserts are normally longitudinally spaced on the buoyancy tanks and molded in place perpendicular to the shrinkage plane. In the cooling phase of the molding process, as the plastic shrinks over the length of the tank, the inserts can""t move with the shrinkage. This results in tank deformation and other problems.
Bulkheads are another factor impacting the manufacture and quality of boat lift buoyancy tanks. Bulkheads are currently used in some existing buoyancy tanks as a divider or baffle to control air and water. Because buoyancy tanks are made by rotational molding, the molded tank is a finished product. Access to the inside of the tank to install a bulkhead is impossible without violating the integrity of the tank. As a result, in known manufacturing processes, bulkhead installation is a time consuming and tedious process.
It is, therefore, an object of this invention to provide a boat lift buoyancy tank which supports all non-plastic connections above the water line when the lift is raised. Another object of this invention is to provide a boat lift buoyancy tank which permits universal selection of connection points for the lift components along the length of the tank. A further object of this invention is to provide a boat lift buoyancy tank which eliminates the need for multiple single position inserts to accommodate connection of the lift components. Yet another object of this invention is to provide a boat lift buoyancy tank which has contours adapted for connection of lift components which will not deform as a result of shrinkage during the molding process. It is also an object of this invention to provide a boat lift buoyancy tank which has an integral internal bulkhead formed during the tank molding process. Still another object of this invention is to provide a boat lift buoyancy tank which eliminates the need for installing bulkheads after the tank is molded.
In accordance with the invention, a plastic rotationally molded polyethylene buoyancy tank for boat lifts allows connection of steel to lengthwise grooves molded into each side of the buoyancy tank. The grooves are located proximate the top of the buoyancy tank so that the steel anchored to the grooves is fully above the water line when the lift is in its raised position. Therefore, the steel is not constantly exposed to the corrosive environment of the water. The lengthwise grooves allow the steel to be attached anywhere along the length of the tank, affording a high degree of flexibility in the design of the boat lift structure.
In one embodiment, a segment of framing steel is bracketed between a pair of inwardly tapered compressible opposed grips. The compressible grips each have outwardly tapered slots with mating tapered rigid inserts. With the inserts partially seated on the grips, the grips are slipped lengthwise or pressed laterally into their respective grooves. Bolts extending through the grips into threaded holes in the inserts are tightened to pull the grips over their tapered inserts. This spreads and compresses the grips in their respective grooves, locking the brackets and connecting steel in place on top of the tanks.
In another embodiment, the framing steel segment or elongated spacer has first and second brackets fixed to each of its ends. First and second elongated members of transverse cross-section are contoured to be seated in each of the lengthwise grooves of the tank. Third and fourth brackets are fixed to each of the first and second elongated members. First and second mechanisms pull the third and fourth brackets toward their respective first and second brackets with the elongated members disposed in their respective grooves so as to lock the spacer in place above the tank. In a satisfactory prototype, the elongated members are tubular, the first and second brackets are angle irons and the third and fourth brackets are channel members. The angle irons and channel members are aligned with their flanges disposed in parallel planes and the webs of the angle irons slide on the webs of their respective channels as the angle irons are pulled toward the channels. The flanges of mated ones of the angle irons and channel members have at least one set of aligned apertures therethrough and the pulling mechanism consists of a plurality of nuts and bolts with one bolt extending through each set of apertures. Fifth and sixth brackets can be fixed between the third and fourth brackets and their respective elongated members to space the third and fourth brackets above their respective elongated members, if necessary. Multiple spacers and associated components can be used on each of the elongated members.
Preferably, a plastic bulkhead is formed within the tank during its molding process. The bulkhead is made from a plastic that has a slightly higher melting point than the tank""s plastic. The plastic bulkhead is held in place in the mold during the tank molding process. The temperature in the process melts the powdered plastic for the tank and almost melts the plastic bulkhead. The two plastics fuse together around the perimeter of the inner mold surface, resulting in a molded-in-place bulkhead.